1. Introduction

Electricity is the fundamental enabler of human development. It permits technological advancements that are reflected in constant growth, while expanding its usage as demands increase. In order to produce electricity, several forms of energy with relatively abundant resources have been harnessed, such as hydroelectric, fossil fuels, and nuclear. However, major economic and sustainability factors throughout history have driven the energy consumption balance toward the exploitation of renewable energies. In fact, as these alternative types of primary energy are available at a variable rate defined by uncontrolled weather, its integration to the electrical network must accomplish a high level of control complexity in order to maximize generation without compromising grid safety.

Traditionally, power systems have had functional topologies that have served convenient routes from bulky generation power plants to load consumption centers. Moreover, distribution systems have been mostly designed radially although there

are also possibilities to transfer circuits to other feeders in case of unscheduled disconnections. Despite the fact that the electrical network was conceived to transport energy vertically from generators to load, it has faced a major challenge to cope with the advent of renewable energies: the bi-directionality of the energy flow [1]. This characteristic aims to provide renewables sources to be distributed across the network in different sizes and at different locations, at the cost of increasing the number of interconnections in the distribution system, introducing new devices, and redesigning existing implementation practices. This paradigm has been fundamental to envisage the concept of smart grid [2], not only because of the energy shift but also the added intelligence the system must have to control such distributed scenario [3, 4]. Additionally, smart grids can contribute to grid survival in the case of natural disasters and large power plant blackouts. Thus, sustainability and safety are concepts that must fit in the smart grid landscape.

But the introduction of local generation, power electronic devices, higher power requirements, and energy storage proved the current grid to be unable to handle all the operational challenges. Therefore, a new highly controllable modular device is needed to comply with the added complexity [14] of the network while maintaining quality standards. The solid-state transformer (SST) has shown to be flexible enough to accommodate several complex functionalities at different voltage levels with the advantage to be lighter and more efficient than the conventional power

In fact, the SST provides the following features: availability of low-voltage DC link, power factor correction, VAR compensation, active filtering, disturbance isolation, and smart protection. The DC link allows the direct injection of distributed renewable energy into the grid. On the other hand, its other features add improved compensation and stability [15] for active and reactive power flow within

The SST concept is not only promising for the smart grid but also for other engineering applications. There have been successful attempts to introduce SST for traction process such as railway transportation, remotely operated vehicles (e.g., submarine applications for deep water exploration), and ship propulsion. Hence, there are a full spectrum of possibilities in which SST has shown to be a feasible alternative, for that reason it is sometimes known as the future "energy router." In order to illustrate the aforementioned capabilities of the SST, this chapter provides an insight into the operation of an SST. Nonlinear loads are revisited as an important part of the SST demand. Then, a mathematical model of an SST is detailed, and its performance under typical power system conditions and disturbances are analyzed. Additionally, a communication feature is also described, such that SST could not only be remotely operated but also take coordinated decisions to optimize power system operation and performance. Therefore, in the next sections, the advantages of SST are studied, demonstrating its feasibility for sustainable

In order to study the benefits of SST in the smart grid, a system model that could represent its electrical properties is needed. Therefore, in this section the mathematical model of nonlinear loads, SST, and some common power system disturbances are described. Later, these models will be used to analyze power system disturbances and highlight the advantages SST has on the network operation.

Concerning the definition of a nonlinear load, it is necessary to specify linearity. Linearity is a characteristic used to describe linear loads, and it corresponds to a property in which loads exclusively produce fundamental sinusoidal current if supplied by a sinusoidal voltage source at fundamental frequency [16]. In contrast, nonlinear loads provide distorted current waveforms, thus injecting harmonic components in the system [17]. Load harmonics higher than fundamental frequency are commonly represented with a resistance-inductance-capacitance (also known as

RLC) circuit in parallel with a current source, as shown in Figure 1.

Nonlinear loads act as sources of harmonic currents whose frequencies are multiple of the fundamental frequency. Harmonics circulated from the load to the source and, depending on the topology of the network, harmonic current can

transformer and its recloser counterpart.

DOI: http://dx.doi.org/10.5772/intechopen.84345

Solid-State Transformer for Energy Efficiency Enhancement

a single smart device.

smart grid applications.

2. Mathematical model

2.1 Nonlinear loads

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Although the distributed energy across the power system has technical advantages so far, much has to be done in order to make it stable and comply with operational and quality standards. There have been several approaches to study reliability improvements [5–8], stability performance [9], communication technologies [10], and several other organizational transformations [11]. As an illustration, under the operational requirements for power system protection, fault-tolerant systems must discriminate the type of failure event based not only on its own measurements but also on its proximity. Thus, integrated communications systems are of uttermost importance in this case [12]. On the other hand, power quality issues must be compensated because other types of phenomena rising from the utilization of new switching technologies based on power electronics will emerge. Hence, maintaining voltage, frequency, and signal cleanliness even during rare extremely low probability events will become a must in new electrical energy devices. If all these conditions are met, the network operator can ensure the stability of the more complex power system. The future smart grid is an intelligent grid with higher levels of reliability and efficiency [13]. Some of the challenges that the smart grid must manage are detailed in Table 1.

The newly demanded performance and functionality mentioned above cannot be obtained with current low frequency power transformers in the grid. These devices transform transmission medium-voltage electrical energy to consumable low-voltage electrical energy at 50/60 Hz frequency. Although it has proven to be highly reliable since power electrification days, it is not designed to handle distributed energy DC production and bi-directional power flow and does not have the capability to handle more complex control other than connection, disconnection, or voltage magnitude control (e.g., tap changers). Nevertheless, the distribution grid had an impulse of intelligence with the deployment of reclosing devices, thus adding more components to the existing infrastructure. This feature has been improved in the last three decades in order to adapt to more stringent conditions.

